In conventional motor vehicles, i.e., automobiles propelled solely by an internal combustion engine, vehicle accessories such as hydraulic pumps, water pumps, vacuum pumps, and heating, ventilation, and air conditioning (HVAC) compressors are powered directly from the internal combustion engine. The power requirements of these accessories may be steady over extended periods of time, as in the case of water pumps used to help cool the engine, or may be required in short bursts, as in the case of hydraulic accessories or air brakes. Regardless, accessory speed is typically proportional to engine speed despite the varying power demand of the accessories.
Conventional automotive accessory drive systems include a drive pulley connected to the engine's crankshaft. A flexible chain or belt couples the drive pulley with a plurality of driven pulleys that are each operatively connected to an individual accessory. In hybrid vehicles which employ an internal combustion engine with one or more electric motors for propulsion, either in series or in parallel, these same accessories (e.g., an electrically-driven A/C compressor or an electrically-driven vacuum pump for a brake booster) are often powered from the internal combustion engine in much the same manner as in conventional motor vehicles.
Since the drive pulley and belt are actuated directly by the crankshaft, they are necessarily subject to engine speed variations during vehicle acceleration and deceleration. In other words, the operating speeds of the accessories in such conventional drive systems are directly proportional to the speed of the engine. Since the engine operates over a wide speed range (e.g., from as low as 500 rpm at idle to as high as 8,000 rpm at full capacity) the accessories are typically designed to be fully functional at the low end of the engine speed range in order to ensure that they can remain operational. Consequently, when the engine is operating at higher speeds, conventional accessory drive systems may transfer more energy to the accessories than necessary to provide adequate function.
The above discussed matter is further complicated for hybrid vehicles with an engine stop-start feature because, under certain operating conditions the engine is shut down to save fuel, yet the accessories need to remain operational. Further, for a belt-alternator-starter type hybrid, it is desirable to be able to start the engine with the alternator-starter without impacting the operation of the accessories.